In high precision spectroscopy measurements the Doppler broadening (spectral lines broadening due to velocity distribution of the particles) is a problem resulting in wide transition line shapes, which are usually too broad and thus not suitable for precision measurements. In order to resolve the naturally narrow spectral features that are hindered by this broad profile it is common to use a spectroscopy configuration employing two counter propagating laser beams.
Polarization spectroscopy is a known Doppler-free experimental technique utilized in high precision spectroscopy. It was first demonstrated by Wieman and Hänsch in 1976 [Wieman and Hänsch, “Doppler-Free Laser Polarization Spectroscopy” Phys. Rev. Lett. 36, 1170, (1976)]. The method is based on analysis of the rotation of the polarization plane of a probe laser beam which is passed through an optically polarized sample by a counter-propagating pump beam [Harris et al., “Polarization spectroscopy in rubidium and cesium”. Phys. Rev. A 73, 062509, (2006)][Do et al., “Polarization spectroscopy of rubidium atoms: Theory and experiment”, Phys. Rev. A 77, 032513 (2008)].
This technique is somewhat analogous to “Saturation spectroscopy” [Demtröder, “Laser Spectroscopy”, (Springer, Berlin 1998)].
In polarization spectroscopy the pump beam is circularly polarized. This circularly polarized pump beam polarizes the sample, resulting in induced anisotropy. The counter-propagating probe beam, which is linearly polarized, travel through the polarized medium in an opposite direction. The change in the angle of the polarization plane of the probe beam due to passage through the polarized sample is analyzed by means of a polarimeter. As the laser frequency is scanned, the Doppler-free spectral features appear as very sharp error signals resulting from the dispersive-like nature of the phenomenon. The better the overlap between the two beams, the sharper the slope of the error signals.
Doppler-free polarization spectroscopy is often used for laser-locking. The main advantages of polarization spectroscopy for laser-locking are that the error signals are produced “naturally” as the laser frequency is scanned over the spectral range of the transitions. This is an advantage compared to saturation spectroscopy, that requires modulating the laser light and which requires additional electronic instruments such as a lock-in amplifier for generating the required error signals.
A conventional setup for Doppler-free polarization spectroscopy is depicted in FIG. 1. In this prior art configuration, the power of the input laser beam 10 is divided into two beams by means of λ/2 waveplate 1 and a polarizing beam splitter cube (PBS—polarizing beam splitter) 2, which splits the beam entering into it into its vertical and horizontal linear polarization components. One component 11 is the probe beam which is reflected by PBS 2 to the sample cell 3. The other linear component 10c of input laser beam 10 is transmitted through PBS 2 and acquires a circular polarization by passing through the λ/4 waveplate 4 to become the pump beam 12. The probe beam 11 passes through the sample cell 3, and the pump beam 12 is directed by mirrors 13 14 and 15 to the opposite side of the sample cell 3 to nearly overlap the counter propagating probe beam 11.
The pump beam 12 polarizes the sample cell 3 and the rotation of the polarization plane of the probe beam 11 due to passing through the polarized sample is analyzed by a polarimeter 19. Polarimeter 19 may comprise a λ/2 waveplate 5 and a PBS 6 which splits the probe beam 11, which becomes circularly polarized as it passes through the λ/2 waveplate 5, into its vertical and horizontal linear polarization components, 11v and 11h. Vertical and horizontal linear polarization components 11v and 11h are then measured by means photodetectors 7.
In polarization spectroscopy the difference between the signals measured by the two photodetectors 7 is amplified by differential amplifier 8 and is measured as a function of the frequency of input laser beam 10. The spectral features of this technique are commonly used as error signals that can serve as input for a feedback system 9 used for locking the laser frequency.
In this configuration the sample atoms which interacts with both the pump and the probe laser beams are only the atoms which do not have velocity components in the direction of propagation of the pump and probe beams, 12 and 11. It is therefore desirous that the pump and probe beams substantially overlap in order to cancel Doppler broadening components. However, as seen in FIG. 1, the pump and probe beams nearly overlap, and inherently, it is not possible to perfectly overlap the pump and probe beams in such conventional polarization spectroscopy setups since there will always be an angle of α degrees between them, which spoils measurements precision.
U.S. Pat. Nos. 3,742,382 and 5,054,921, describe similar Doppler-free configurations. These prior art Doppler-free configurations do not enable perfect overlap between the pump beam and the probe beam. As will be appreciated by those skilled in the art, in order to reduce the angle α between the pump and the probe beams and increase the beam overlap, it is required to extend the length of such conventional Doppler-free configurations and therefore to increase the dimensions of the apparatus. Such a configuration is not ideal for miniaturization and alignment. Additional difficulties arising in such conventional spectroscopy setups are due to the multiple mirror elements comprised therein that require careful alignment, and thus complicates the assembly of devices based on it.
Polarization spectroscopy configuration can also be used to measure magnetic fields with high accuracy using a phenomenon commonly known as Nonlinear Magneto Optical Rotation (NMOR) [Budker and Romalis, “Optical Magnetometry” Nature Physics, 3, 227 (2007)][Budker et al., “Resonant nonlinear magneto-optical effects in atoms”, Reviews of Modern Physics, 74, 1153 (2002)]. In NMOR, the medium is spin polarized due to the difference between absorption coefficients (circular dichroism) of the left and right circular polarization components. The medium dichroism causes the polarization of the probe beam to become elliptical, and the optical birefringence (Δn=n+−n−, where n+ and n− are the refractive indices for right and left circular polarizations) induces the rotation of the axis of polarization of the linearly polarized probe beam by Δθ=Δnk0L/2, where k0 is the wave number of the probe beam, L is the length of the sample cell, and Δn is the optical birefringence. Accordingly, the magnitude of magnetic fields can be detected and accurately measured by exposing the sample (i.e., atomic vapor) to the magnetic field to be measured.
On-resonance NMOR detection causes excitation of the medium and changes the properties of the state to be measured. Therefore the most sensitive up-to-date magnetometers use a probe beam that is detuned far from the resonance frequency. Off-resonance detection is compatible with high probe-beam intensities, which improve the signal-to-noise ratio. NMOR is also often operated in a pump-probe configuration [Kominis et al., “A subfemtotesla Multichannel atomic magnetometer”, Nature, 422, 596 (2003)], as in polarization spectroscopy. In such NMOR setups the pump beam polarizes the sample and the resulting magnetization process around the direction of the magnetic field and is detected by the probe beam. However, in most cases where NMOR is performed in a pump-probe fashion the beams are perpendicular to each other [Kominis et al., Nature, 422, 596 (2003)], hence the sensitivity is limited by the intersection volume of the beams. Furthermore, in the common pump-probe NMOR setups, in which the beams are perpendicular to each other, it is necessary to have optical access to the vapor cell from two directions which present a difficulty when considering scalability in applications requiring array of such pump-probe NMOR cells (e.g., magnetic imaging).
Coherent Population Trapping (CPT) is a known resonance phenomenon occurring due to a quantum mechanical interference effect in an atomic/molecular system that cancels the absorption of a coherent bichromatic light field [Wynands and Nagel, “Precision spectroscopy with coherent dark states” Appl. Phys. B 68, 1-25 (1999)]. The bichromatic light field is comprised of two phase coherent optical frequencies, and wherein the frequency difference between these two fields matches the ground state splitting in the sample there is a change in the intensity of the light transmitted through the sample. This change is strongly dependent on the frequency difference between the two phase coherent fields. CPT proved to be useful for various application in areas as diverse as laser cooling [Metcalf and der Straten, “Laser cooling and trapping of atoms” J. Opt. Soc. Am. B 20, 887 (2003)], frequency standards [Vamier, “Atomic clocks based on coherent population trapping: a review”, Appl. Phys. B 81, 421, (2005)][Vanier et al., “Coherent population trapping in cesium: Dark lines and coherent microwave emission” Phys. Rev. A 58, 2345 (1998)], high-sensitivity optical magnetometry [Nagel et al., “Experimental realization of coherent dark-state magnetometers”, Europhys. Lett., 44, 31-36 (1998)][Schwindt et al., “Chip scale atomic magnetometer” Appl. Phys. Lett. 85, 6409 (2004)] and light storage [Phillips et al., “Storage of light in atomic vapor” Phys. Rev. Lett. 86, 783-786 (2001)]. In many cases the term Electromagnetic Induced Transparency (EIT) is used to describe the same phenomenon. It is also important to note that in most cases, CPT signals are weak (reduce the absorption signal by ˜1-2%) due to radiative decay of the atomic population to “extreme” states which are not coupled with the two phase coherent light fields, which are often called trapped states.
CPT is commonly applied on a sample cell containing a vapor of alkali metal. In general, the ground state hyperfine splitting in alkali atoms is in the microwave range (˜6.8 GHz in Rb, 9.2 GHz in Cs). To produce two coherent light fields with such a frequency difference it is common to modulate the laser current with a microwave source. Coherent population trapping (CPT), has applications in areas as diverse as laser cooling frequency standards and high-sensitivity magnetometry.
For completeness, the accuracy of the above mentioned spectroscopy techniques depends on the coherence lifetime of the atoms which in practice is reduced by a variety of nonradiative processes such as atom-atom collisions or collisions with the vapor cell walls. Hence, to reduce the decoherence rate of these processes the vapor cell inner walls are sometimes coated by a substance such as paraffin which maintains the spin coherence of the atoms even after many collisions. To reduce decoherence rate due to collisions between alkali atoms, often the cell is filled with high pressure inert gas such as He, Ar etc.
The approaches described above do not provide satisfactory solutions for high precision Doppler-free spectroscopy. Therefore there is a need for improved precision Doppler-free spectroscopy configurations.
One object of the present invention is to provide high precision Doppler-free polarization spectroscopy configurations suitable to be used as atomic references.
It is also an object of the present invention to provide high precision Doppler-free polarization spectroscopy configurations in which Doppler-broadening effects are substantially minimized or even entirely cancelled.
It is another object of the present invention to provide a method and apparatus for high precision Doppler-free polarization spectroscopy allowing for significant miniaturization of the apparatus, and that can also be used for magnetometry applications.
It is a further object of the present invention to provide a method and apparatus for optically pumped magneto optical rotation magnetometer with improved optical access.
It is yet a further object of the present invention to provide a method and apparatus for high precision Doppler-free polarization spectroscopy that allows combining laser locking with CPT measurements.
An additional object of the present invention is to provide a method and an apparatus for improving the contrast of a CPT signal.
Other objects and advantages of the invention will become apparent as the description proceeds.